1. Field of the Invention
This invention relates to cameras, and more particularly to the automatic rewinding of film and the detection of the voltage of an electrical power source.
2. Description of the Prior Art
In the conventional camera, as the film exposure has completed, when to wind up the film, if the film is not wound up by one frame by a motor for film winding up despite the control circuit is producing the drive signal for winding up film in a state that the film winding up is not completed, the state for performing the film winding up is automatically changed over to a state for performing rewinding of the film. Many cameras having such a so-called auto-U-turn capability are known. However, in the case where despite the drive signal for winding up the film is being produced, the motor for film winding up does not rotate, the unequivocal performance of the auto-U-turn produces the following drawback. That is, even in a situation that despite the film is not photographed to a prescribed number of frames, because the capacitance of the electrical power source or battery is insufficient, the motor for winding up stops on the way to wind up the film, the auto-U-turn takes place. Such a drawback has been produced.
To eliminate such a drawback, a method may be considered that when the motor for film winding up has stopped, the voltage of the battery is detected, and only when said voltage has been above a prescribed value, the auto-U-turn is caused to take place, and that if it does not reach the prescribed value, the auto-U-turn is hindered. However, when the motor for film winding up has stopped, because a so-called rush current corresponding to the internal resistance of the coil of the motor flows thereto, a large load is added to the battery, and the voltage drops. It takes a long time to recover the battery voltage. Therefore, even in a state that the capacity of the battery sufficiently exists, there is an occasion that the voltage is lowered to the neighborhood of the above-described prescribed voltage. In order to accurately discriminate the state of the electrical power source or battery, a voltage setting having a margin to some extent must be made. Or otherwise, in practice, even for a battery whose capacity is left sufficiently to be usable, it would be unavoidable to discriminate it as no longer usable. Also, it may be considered to wait a time for recording the battery. But the elongation of the control response time is unconvenient to the user and it has sometimes been mistaken for damage.
Also, meanwhile, in U.S. Pat. No. 4,458,996, a control circuit of camera is disclosed that after the film has been exposed, and a shutter closing signal is produced, whether or not the battery of the electrical power source is sufficiently present is detected, and if so the motor for winding up the film is driven. In the camera disclosed in such publication, nothing is disclosed about the auto-U-turn that in the film winding up state, when the film is not wound up by one frame by the film windup motor despite a signal for winding up the film is being produced from the control circuit, the film is automatically rewound.
Next, in the above-described detection of the electrical power source or battery, in general, the cameras have dry batteries as the electrical power source. In order to detect that the capacity of such an electrical power source is sufficient, the internal resistance of the battery must be taken into account. So it is nonsense to do detection without connecting a load to the battery. Therefore, there was need to connect the load to the battery when the battery voltage is detected. A first method of detecting the battery voltage with the load connected to the battery was to provide a load resistor solely usable for battery voltage detection within the camera. A second method was to use a magnet for performing the photographic operation of the camera, for example, the aperture control or shutter control commonly as a load for battery voltage detection.
By the way, in the recent cameras, there are increasing occasions that not only the electromagnet is used for controlling the diaphragm or shutter, but also a motor is used for winding up the film or charging the the diaphragm and the shutter. So, as the electrical power source, not a battery of small size such as mercury battery as in the past, but a relatively large size battery, for example, AA type battery is used. Therefore, in the recent cameras, in order to detect whether or not the capacity of the electrical power source is sufficient, a load of large power capacity becomes necessary. To employ the above-described first method, it is required that the solely used load resistor be made a resistor of large power capacity. Not only this causes the cost to rise, but also, because such a resistor becomes a resistor of large size, it is near the impossibility to find a space for its installation in such an instrument of small size as the camera.
Also, in the case when the above-described second method was employed, because it leads to actually supply current to the magnet for performing the photographic operation of the camera, there was need to provide an additional mechanical member for inhibiting the release of, for example, a latch for the shutter so that even when the magnet for performing the operation of the shutter was energized for the purpose of detecting the battery voltage, the shutter did not open.
By the way, in the conventional cameras, as the magnet for performing the photographic operation of the camera, use aas made of the so-called current hold type magnet in which the armature was attracted during the time when current was supplied thereto, and was released when the current supply was cut off. So, there was provided a member for mechanically holding the armature when such a magnet was not energized. Therefore, in the camera using the conventional current hold type magnet, the above-described second method could readily be employed.
However, because the current hold type magnet held the armature by attraction so long as the current was supplied thereto, there was a drawback that when such a magnet of the current hold type was used for controlling, for example, the running down of the trailing curtain of the shutter, as the shutter time was set to a long value, for example, 10 seconds, the consumption of electrical energy necessary to energize the magnet for so long a time was large.
Therefore, in the recent cameras, a combination type of magnet with a permanent magnet incorporated in the solenoid is finding increasing use.
Such a magnet holds the armature by the attraction of the permanent magnet when the solenoid is not energized. By supplying current to the solenoid, as the magnetic flux of the permanent magnet is cancelled by the magnetic flux of the solenoid, the armature is released. Therefore, an advantage of reducing the consumption of electrical energy was produced, but there was provided no member for mechanically holding the armature when the solenoid was unenergized as in the current hold type magnet.
Therefore, in the camera using many magnets of the combination type, the above-described second method could not be employed. Also, to specially provide a member of mechanically holding the armature for the purpose of detecting the battery voltage could not really be made from the point of view of the cost and space. Even if it could, there was a drawback that such a magnet was too small as the load for the battery.
As has been described above, in the recent cameras, there was a drawback that in order to detect whether or not the capacity of the electrical source was sufficient, any of the above-described first and second methods was not suitable.